Monopulse radar antenna array feed network

ABSTRACT

An array antenna system and feed network providing sum-anddifference monopulse transmission and reception. To permit independence of its sum and difference excitations, the array may be divided into four or more separate subarrays which are arranged in pairs symmetrical with respect to the center of the array and interconnected through sum and difference couplers selected to optimize the sum beam gain and the difference beam angular sensitivity. To further optimize the difference beam pattern, one or more of the array elements on each side of and nearest to the array center are intercoupled through a &#39;&#39;&#39;&#39;meaner&#39;&#39;&#39;&#39; network which at least partially cancels the antiphased difference excitation for these elements while leaving their equiphased sum excitation unchanged, to thereby lower the difference beam sidelobe level without detriment to the sum beam pattern.

O United States Patent [191 Kinsey Apr. 9, 1974 MONOPULSE RADAR ANTENNA ARRAY FEED NETWORK 57 AB TRACT Inventor! Richard Kinsey Dewltt, An array antenna system and feed'network providing [73] Assigneez General Electric Company, sum-and-difference monopulse transmission and re- Syracuse, ception. To permit independence of its sum and difference excitations, the array may be divided into four or Flledl P 1972 more separate subarrays which are arranged in pairs [21] Appl. No.: 285,861 symmetrical with respect to the center of the array and interconnected through sum and difference couplers selected to optimize the sum beam gain and the [1.8. CI. diffe be m angular sensitivity To further pti- Cl. mize the differen e beam pattern one or more of the [58] held of Search 343/778 100 LE array elements on each side of and nearest to the array center are intercoupled through a meaner net- [56] References Clied work which at least partially cancels the antiphased UNITED STATES PATENTS difference excitation for these elements while leaving 3,392,395 7/1968 Hannan 343/755 their equiphased Sum excitation unchanged to 3,618,092 11/1971 Waineo.. 343/853 thereby lower the difference beam sidelobe level with- 3,293,648 12/1966 Kuhn 343/854 out detriment to the sum beam pattern.

Primary ExaminerEli Lieberman 3 Claims, 6 Drawing Figures 3 COUPEER IATENTEDAPR 9 I974 COUPLER 23 3dB I? HYBRID A J j 3dB HYBRID \l9 2 COUPLER II VI H\V||| I V'lll VII VII VIII- 3 n m V A 1 BR H F m v R W III VI VI VII. VII VIII VI VII RI R2 R3 R4 R5 R6 R7 R8 R9 RIO RII RI2 A COUPLER 2 COUPLER R24 R23 R22 R2l R20 Rl9 RIB RI? RIG Rl5 RI4 ATENTEI] APR 9 I974 TT T SHEET 3 OF 3 FIG.6

3dB OUPLER MW T 3dB COUPLER 3dB YBRID COUPLER 3dB COUPLER 3GB COUPLER v 1 B LER W W T T T T 3dB YBRI MONOPULSE RADAR ANTENNA ARRAY FEED NETWORK BACKGROUND OF THE INVENTION This invention relates generally to array antennas and feed networks therefor, and more particularly to such antennas and feeds for use in monopulse radar systems providing both sum and difference signals to the associated monopulse receivers.

The particular adaptability of array antennas to monopulse radar operation has long been recognized. Commonly such antennas comprise a number of individual radiation elements such as waveguide slots or dipoles arranged in ordered array, to form a single linear array or a planar array made up of a plurality of stacked linear arrays. The signals received by the elements of each linear array are combined in a pair of feed networks which may be of either series feed or corporate feed configuration and which are disposed symmetrically with respect to the antenna boresight to form with their associated radiation elements two identical half apertures. For monopulse reception the two half aperture signals thus provided are added to each other to produce the sum signal output, and subtracted from each other to yield a difference signal output constituting a measure of target angle off boresight in the plane of the linear array from which the signal derived.

In designing an antenna array of this kind for monopulse application the normal objectives are to maximize the sum beam gain and the difference beam angular sensitivity, and to minimize the sidelobe levels of both the sum and difference beams. With elemental feed networks as just described it is not possible to optimize all these performance characteristics simultaneously, however, because their interdependence permits the optimization of any one of them only by compromise of one or more of the others.

To avoid the need for such compromise various antenna array feed networks capable of producing independent sum and difference excitations have been proposed. For example, U.S. Pat. No. 3,509,577 issued to the present applicant describes a tandem series feed system providing substantial independence of sum and difference excitations, thus enabling design for low sidelobes for both sum and difference beams as well as good sum beam gain and difference beam angular sensitivity.

However, the tandem series feed constitutes a more elegant solution to the problem of achieving independence of sum and difference excitations than is required to meet the requirements of some applications, in which a lesser degree of independence between excitations would be adequate and would represent an acceptable trade-off against feed complexity. One known array feed providing such limited or partial independence of sum and difference excitations divides the elements of each linear array into two or more subarrays per half aperture, with partial independence then being afforded by the ability to separately control the sum and difference power split to each of the subarrays and also by control of the number of array elements included in each of the subarrays.

The degree of independence of the sum and difference excitations obtainable by division of the array into subarrays in this manner depends upon the number of subarrays into which division is made, but as a practical matter the number must be kept very small in order to avoid undesirable complexity of the array feed network structure. With even the smallest possible number of subarray groupings, i.e., two subarrays per half aperture, sufficient independence of sum and difference excitations will usually be available to enable acceptable degrees of optimization of three of the four performance characteristics previously mentioned, viz., sum beam gain, sum beam slidelobe level, and difference beam angular sensitivity. The fourth parameter, the difference beam sidelobe level, cannot also be brought in this way to as low a level as would be desirable, and difference beam sidelobe level thus remains a potential problem for many monopulse radar applications for which this form of feed network would otherwise be attractive.

The present invention is directed to this problem of difference beam sidelobe level, and it provides a solution through which a very substantial reduction of difference beam sidelobe level may be achieved with no adverse affect on either the sum beam gain or the sum beam sidelobe level, and with little if any affect on the difference beam angular sensitivity. The feed network modification by which this is'accomplished lends itself to application to antenna arrays of both series feed and corporate feed types, and is relatively simple and inexpensive in implementation and thus adds little to system total cost.

BRIEF SUMMARY OF THE INVENTION In accordance with the invention, a substantial improvement in the difference beam sidelobe level in a monopulse array antenna is achieved by intercoupling one or more of the array elements nearest its center through a meaner element which effectively operates to at least partially cancel the difference signals coupled to and from those array elements, while dividing the sum signals equally between the array elements thus coupled. As a result the antiphased difference excitation of the coupled center element is substantially reduced or made zero, thus better conforming the difference excitation values for those elements to the optimum difference excitation for low difference beam sidelobe levels. The equiphased sum excitation of the coupled center elements remains unchanged by their intercoupling in this manner, so the sum beam pattern is not adversely affected thereby.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elementary block diagram of an antenna array and feed network providing partial independence of sum and difference excitations in accordance with the prior art;

FIG. 2 is an elementary block diagram of a similar.

DESCRIPTION OF THE PREFERRED EMBODIMENT With continued reference to the drawings, FIG. 1 illustrates a prior art monopulse radar antenna array with corporate feed, in an arrangement providing partial independence of sum and difference excitations enabling improved difference pattern angular sensitivity without penalty to sum pattern performance. As shown, the system comprises a plurality of radiation elements ll grouped into paired subarrays 13a, 13b, and 15a, 15b, with the subarrays each comprising a number of array elements and being disposed symmetrically with respect to the array center. The two subarrays 13a and 13b nearest the array center connect through a 3dB hybrid 17, and the two outer subarrays 15a and 15b similarly connect through another 3dB hybrid 19, to sum and difference couplers 21 and 23. The other ports of these couplers connect to sum and difference terminals and 27, respectively, or are terminated as shown.

In monopulse operation, the transmitter and the sum receiver connect through suitable duplexing means (not shown) to the sum terminal 25, and the difference receiver connects to the terminal 27. Transmitter pulse energy applied at terminal 25 divides in coupler 21 and is applied, in predetermined ratios not necessarily equal, to the sum terminals of each of the 3dB hybrids 17 and 19. Here the pulse energy is equally divided in each of the hybrids and applied half to each of the two subarrays to be supplied through that hybrid, thus energizing the entire array with a splut of power as between the inner subarrays 13a, 13b and the outer subarrays 15a, 15b which is determined by design of the sum coupler 21.

On reception, the reflected signal energy received by the inner subarrays 13a and 13b combines in hybrid 17 to yield both a sum signal which is applied to coupler 21 and a difference signal which is applied to coupler 23; reflected signal energy received by the outer subarrays 15a and 15b similarly combines in hybrid 19 to yield a sum signal which is applied to coupler 21 and a difference signal which is applied to coupler 23. The sum signals add in coupler 21 to yield a sum signal output on terminal 25, and the difference signals add in coupler 23 to yield a difference signal output on terminal 27.

In designing an array providing partial independence of sum and difference excitations as in FIG. 1, the usual procedure is to choose the element excitation and sum beam coupler values so that the sum beam will have the desired gain and sidelobe characteristics. The difference beam coupler value, and the number N, of radiation elements 11 to be included in each of the inner subarrays 13a and 1312, then are determined so as to maximize the difference beam angular sensitivity. Of course, another parameter which may be adjusted for optimization of the difference beam angular sensitivity is the number of subarrays into which the array is divided, though unless this number is kept quite small much of the attractive simplicity of the basic feed network is lost.

The remaining performance characteristic which array design formulation in accordance with the foregoing does not satisfactorily optimize for many monopulse applications is difference beam sidelobe level. As shown by the factor patterns for a representative such array in FIG. 3, the difference beam sidelobes are much higher than the sum beam sidelobes, with the peak sum sidelobes being below -25dB while the peak difference sidelobes are down only -l7.7dB in this particular example, which is an undesirably high sidelobe level in many cases.

Referring now to FIG. 2, there is shown a monopulse array like that of FIG. 1 except that it embodies center element intercoupling so as to yield substantial improvement in difference beam sidelobe level in accordance with the invention. As in FIG. 1, the radiation elements l 1 which are further designated R R in FIG. 2, are again grouped into four subarrays. These subarrays are connected through hybrids 17 and 19, and couplers 21 and 23, to the sum and difference terminals 25 and 27 as previously described. Also as described, the radiation elements 11 of the outer subarrays 15a and 15b are fed through a conventional corporate feed arrangement comprising couplers C, C but the feed network for the inner subarray elements is modified to provide intercoupling of the two center elements of the array through a pair of 3dB couplers 29 and 31. As shown, coupler 29 has its 3dB terminals connected to the centermost radiation elements R, and R its difference port connected to a terminating load, and its sum port connected to the sum port of coupler 31, which has its 3dB ports then connected into a corporate feed network otherwise similar to the subarray networks 13a and 13b of FIG. 1.

The operation of these couplers 29 and 31 is to cancel the difference signal energy which is coupled through them, and to add and then divide the sum signal energy equally between their two inputs and outputs, in both directions of signal energy flow. The couplers thus function as a meaner of signal energy coupled to and from the centermost radiation elements 11, the effect being to leave unchanged the equiphased sum excitation while cancelling the antiphased difference excitation.

The improvement in difference beam sidelobe level accomplished by this modification is illustrated in FIG. 4, which depicts factor patterns for an array like that of FIG. 1 except for the modification just described, and shows the difference beam peak sidelobe level to have been dropped to approximately 21.9dB, for a net improvement of about 4dB. There may in some cases be some accompanying reduction in angular sensitivity but in general this is negligible in amount, being only about 0.07dB in the case of the particular array being described, and such slight angular sensitivity loss is far more than offset by the improvement achieved in difference beam sidelobe level.

The reason for the pattern improvement thus obtained can be explained in several different ways. For present purposes it appears sufficient to note that the effective cancellation of the difference beam excitation of the centermost elements better conforms the overall difference beam excitation to that required for optimum difference beam sidelobe level. This is illustrated in FIG. 5, in which the optimum difference excitation is shown by the solid line curve, and element weights for the array of FIGS. 1 and 2 are shown as small circles above the corresponding radiation element numbers. As indicated, the difference excitation for the centermost elements, the elements numbered R and R in the drawings, is reduced from its normal value to zero by the feed network modification just described, and the result of this change is better conformance of the effective difference excitations for the elements to the optimum excitation curve shown in FIG. 5.

To complete the description of the specific exemplary embodiment of which FIGS. 3, 4 and 5 are illustrative. the following tabulation is given of coupler values and sum and difierence weights used in this system:

TABLE I Coupler dB Cl, Cll 3.0l C2, C12 3.0l C3, C13 3.01 C4. C14 3.0l C5, C15 3.01 C6. C16 3.01 C7, C17 3.30 C8, C18 3.20 C9, C19 4.00 C10, C20 5.50 ZCoupler 21 4.30 ACoupler 23 6.00

TABLE II Element Number Sum Weights Delta Weights R R 1.000 0.632 (without intercoupling) 0.000 (after intercoupling) R,, R 0.761 L080 In FIG. 2 only the centermost radiation element of each half aperture is shown intercoupled through the meaner" network. In some cases, particularly with very large arrays, it may be desirable to couple two or more of the radiation elements of each half aperture through the meaner network, where to do so would better conform the difference excitation to the optimum for that specific array. Also, if the array contains an odd number of radiation elements rather than an even number as illustrated, a conventional three-way divider may be substituted for the coupler 29 with each of its three branch ports connected to one of the three centermost radiation elements.

Other options in applying the pattern improvement technique of this invention include the substitution of other types of intercoupling networks for the particular two-hybrid network 29-31 shown. As explained, this network functions simply to cancel the difference signal energy and to mean the sum signal energy passing either way between its input and output ports, so other known devices such, for example, as the so-called split T hybrid which may be arranged to function in the same general manner may be substituted. For some applications it may be preferable that the intercoupling network only partially cancel the center element difference signal energy, rather than reduce it to zero as in the embodiment of FIG. 2. Partial cancellation may in such cases enable significant reductions in difference beam sidelobe level while affecting angular sensitivity to a still smaller extent than with complete cancellation as previously described.

A system incorporating center element intercoupling thus modified is shown in FIG. 6, in which the system illustrated is otherwise like that of FIG. 2. Here the "meaner network 29-31 has the right-hand 3dB branch of its coupler 29 connected to couple half the signal energy to and from the pair (R R of radiation elements 11 nearest the array center on its right-hand side, such coupling being accomplished through a second meaner network 33-35 connected for operation in the same manner as previously described for network 29-31. The left-hand branch of coupler 29 couples in like manner through a third such meaner network 37-39 to the left-hand pair (R R of array elements 11 nearest the array center.

In operation, the sum pattern again remains essentially unchanged, because sum signal energy propagating through the combined meaner network 29-39 is not significantly affected thereby. Signal energy received by elements R and R is summed in coupler 33, then divided by coupler 35 with one half the sum signal energy being transmitted directly to coupler C22 and the other half being transmitted thereto through the meaner network 29-31. Sum signal energy is similarly coupled through networks 37-39 and 29-31 from array elements R and R to coupler C21.

Substantially all the difference signal content of signals receivedby the two radiation elements of each array element pair R R and R R is cancelled by operation of the couplers 33 and 37 to which these pairs respectively connect. The sum signal outputs of couplers 33 and 37 are transmitted to couplers 35 and 39, respectively, and there each is split with half the signal energy being applied directly to coupler C21 or C22 and half applied thereto through the meaner" network. These half-level signals for the leftand righthand array element pairs which are applied to network 29-31 have their difference content cancelled thereby, but the difference content of the half-level signals applied directly to couplers C21 and C22 is preserved and is factored accordingly into the difference signal output through couplers C23 and C24, hybrid 17 and A coupler 23.

The net result is that the amplitude of the difference signal for the two centermost pairs of array elements R R and R R is reduced by one-half its normal value, rather than cancelled completely as in the embodiment of FIG. 2. Such reduction in amplitude of the difference excitation applies on transmit as well as receive, by reason of the reciprocity theorem, and again is accomplished without significant affect on the sum excitation.

While the invention has been shown and described as applied to a system in which the array feed network is of corporate type, it will be appreciated that the invention is applicable as well to systems utilizing series feed of the array elements. Also, while the invention has been described in its application to systems providing partial independence of the sum and difference excita tions, it may be applied to advantage even in systems not affording this capability. In such systems, in which the array half apertures would not be further subdivided, one or more of the centermost elements of each half aperture would be intercoupled through a meaner network just as described hereinbefore.

These and many other modifications of the exemplary embodiment of the invention described will occur to those skilled in the art and it therefore should be understood that the appended claims are intended to cover all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. For use in a monopulse transmitting and receiving system, an array antenna and feed network comprising:

a. a plurality of radiation elements ordered in linear array;

b. feed means connecting said radiating elements to form at least two pairs of subarrays with the subar ray pairs symmetrically disposed with respect to the center of the array;

c. hybrid junction means having sum and difference signal ports, said hybrid junctions being respectively coupled to each of said subarrays to provide sum and difference signal modes respectively at said sum and difference signal ports;

d. sum signal coupler means and difference signal coupler means respectively connected to the sum signal ports and difference signal ports of said hybrid junctions for sum signal input to the array and sum and difference signal output therefrom; and

e. a meaner element intercoupling one or more of array;

b. feed means connecting said radiating elements to form two half apertures disposed one on each side of the array center;

c. sum and difference signal coupler means connected to said feed means for sum signal input to the array and sum and difference output therefrom; and

d. a meaner network intercoupling said feed means and a plurality of said radiation elements including at least one of the centermost of said radiation elements on one side of the array with a like number on the other, said meaner network comprising first coupler means wherein the radiation element sum and difference signal intercoupling is such as to at least partially cancel the antiphased difference signals and to add the equiphased sum signals, and second coupler means wherein the equiphased sum signals thus added in said first coupler means are divided to yield a corresponding plurality of sum signals, whereby the antiphased difference excitation of the array center elements is at least partially cancelled while the equiphased sum excitation thereof is maintained substantially unchanged.

3. An array antenna system as defined in claim 2 wherein said meaner" network is connected to two of said centermost radiation elements on each side of the array and operates to reduce by one-half the amplitude of the difference excitation for those elements. 

1. For use in a monopulse transmitting and receiving system, an array antenna and feed network comprising: a. a plurality of radiation elements ordered in linear array; b. feed means connecting said radiating elements to form at least two pairs of subarrays with the subarray pairs symmetrically disposed with respect to the center of the array; c. hybrid junction means having sum and difference signal ports, said hybrid junctions being respectively coupled to each of said subarrays to provide sum and difference signal modes respectively at said sum and difference signal ports; d. sum signal coupler means and difference signal coupler means respectively connected to the sum signal ports and difference signal ports of said hybrid junctions for sum signal input to the array and sum and difference signal output therefrom; and e. a ''''meaner'''' element intercoupling one or more of the centermost of said radiation elements on one side of the array with a like number of the centermost elements on the other side thereof, said ''''meaner'''' element being operative to cancel the difference signal energy transmitted therethrough and to divide the sum signal energy equally between the radiation elements intercoupled therethrough.
 2. An array antenna system providing sum-and-difference monopulse transmission, comprising: a. a plurality of radiation elements ordered in linear array; b. feed means connecting said radiating elements to form two half apertures disposed one on each side of the array center; c. sum and difference signal coupler means connected to said feed means for sum signal input to the array and sum and difference output therefrom; and d. a ''''meaner'''' network intercoupling said feed means and a plurality of said radiation elements including at least one of the centermost of said radiation elements on one side of the array with a like number on the other, said ''''meaner'''' network comprising first coupler means wherein the radiation element sum and difference signal intercoupling is such as to at least partially cancel the antiphased difference signals and to add the equiphased sum signals, and second coupler means wherein the equiphased sum signals thus added in said first coupler means are divided to yield a corresponding plurality of sum signals, whereby the antiphased difference excitation of the array center elements is aT least partially cancelled while the equiphased sum excitation thereof is maintained substantially unchanged.
 3. An array antenna system as defined in claim 2 wherein said ''''meaner'''' network is connected to two of said centermost radiation elements on each side of the array and operates to reduce by one-half the amplitude of the difference excitation for those elements. 